A systematic approach to unconventional play analysis: the oil and gas potential of the Kockatea Shale and Carynginia Formation, North Perth Basin, Western Australia

Abstract

Key formations throughout the North Perth Basin have been mapped from 3D and 2D seismic data to define depth grid inputs to a 3D basin model calibrated with temperature and maturity data from 45 wells, plus an additional 27 pseudo well models. The Permian Carynginia Formation and Early Triassic Hovea Member of the Kockatea Shale have been defined in this model as unconventional shale reservoir targets. Basin-wide pyrolysis data have been used to construct kinetics curves for both the Carynginia Formation and Kockatea Shale, which define Type D/E and mixed B, and D/E kerogen types, respectively. When combined with thermal history inputs, these source rocks expel and retain significant volumes of hydrocarbons, of which the free hydrocarbons in the retained components reach 22 BCF/km2 for the Carynginia Formation gas and 8 MMBBLS/km2 and 21 BCF/km2 for the Hovea Member liquids and gas, respectively. The defined kinetics relationships allow the estimation of kerogen-specific oil and gas windows, which have been applied across the study area to map unconventional play fairways for both formations, and to calculate the initial total organic carbon (TOC) and hydrogen index (HI) for each unit prior to significant maturation. This study employs a mass balance approach through basin modelling as a means of estimating likely retained hydrocarbon volumes in key unconventional reservoirs in the basin. Sonic and density data from 28 wells in the basin have been used to calculate theoretical porosity to determine likely areas of overpressure. When combined with observed connection gas peaks and modelled maturity, there is a reasonable correlation suggesting that the basin exhibits modest overpressure of 2–6 MPa associated with the main gas window at 1.2 Ro% and this observation is applied to the play fairway mapping process. Play fairways are further constrained through geomechanical and stress considerations from mechanical earth models (MEMs) built from log and image data for wells in the basin. These data define an overall strike-slip stress regime with SHmax consistently oriented east to west with the exception of local perturbations. Dynamic rock strength calculated from the same MEM process shows target zones in the Kockatea Shale and Carynginia Formation ranging from ~60–130 MPa unconfined compressive strength (UCS), calibrated against available static data. The net thickness of rock with a UCS >75 MPa is mapped and overlain on retained in place hydrocarbon maps to restrict the area of likely economically extractable resource. While unconventional play cut-offs in the Perth Basin are notably lower than those commonly used in shale gas plays in the US, successful stimulation of Perth Basin rocks has been demonstrated by substantial flows from wells such as Arrowsmith–2. This study outlines a new workflow for mapping unconventional resources and suggests that Australian rocks are unique in both depositional environment and mechanical properties such that unconventional assessment using US play cut-offs may be misleading.